1. Field of the Invention
The present invention relates to an improved continuous process for preparing polysilicate microgels whereby silica deposition is reduced or eliminated.
2. Description of Related Art
Polysilicate microgels (i.e., aqueous solutions formed by the partial gelation of an alkali metal silicate) are well known in the art. Partial gelation typically is achieved by mixing an alkali metal silicate with a gel initiator, aging the mixture for a short time, and then stopping gelation by diluting the mixture. Mineral acids and alum are the most commonly employed gel initiators. Resulting microgels have commercial utility as a drainage and retention aid in paper making, as a flocculation agent in potable water purification plants, and in similar applications.
Several practical factors currently limit commercial use of polysilicate microgels, although they are excellent flocculents and environmentally benign. For example, microgel solutions necessarily are dilute, making it impractical to ship large volumes long distances. Microgels also are prone to gel and to form silicate deposits in equipment used to prepare the product. These problems can be overcome by equipment design and trained personnel in a factory environment, but present greater difficulty in field applications where the equipment should be relatively easy to operate and maintain.
Batch process for producing microgels age the microgel in large mixing and holding tanks, which not only are expensive but also introduce the problems of product nonuniformity and process control inherent in a batch process. A continuous process to produce and age the microgel is much preferred for consistency in product quality. Unfortunately, silica deposits can be tolerated better in batch holding tanks than in the plumbing of a continuous process, where the deposits plug the equipment causing frequent shut-down for maintenance.
U.S. Pat. Nos. 5,279,807; 5,503,820; and 5,658,055 disclose improved continuous processes for preparing polysilicate microgels wherein silica deposition is greatly reduced by mixing the soluble silicate solution and gel initiators under specific conditions. While the design taught in these patents results in much improved performance, and has found commercial utility, silica deposits still frequently occur that clog the pipes and instruments of the microgel generator. Thus, it remains necessary to periodically shut down the process and purge the deposits by dissolving them with a base, such as sodium hydroxide. It also remains necessary to have a stand-by generator if continuous production is required.
Accordingly, there is a need for further improvements to the continuous process for preparing polysilicate microgels.